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Avalanche Forecast

Archived

Feb 29th, 2020–Mar 1st, 2020

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

Sea To Sky.

Don't let clear skies create complacency with a complicated snowpack. A tricky pattern of wind-drifted snow and an atypical buried weak layer require careful evaluation and terrain selection on Sunday.

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

Saturday night: Clear, light northwest winds, freezing level dropping to valley bottom.

Sunday: Mostly clear, increasing cloud in the afternoon, light variable winds, freezing level 1000 m. 

Monday: Increasing cloud, isolated flurries with trace accumulations, light southwest winds, gusting strong at ridge-top, freezing level 1100 m.

Tuesday: Mostly cloudy, 10-15 cm of snow, moderate southwest winds, gusting strong at ridge-top, freezing level rising to 1300 m. 

Avalanche Summary

By Friday afternoon, small (size 1-1.5) wind slabs were easily human-triggered on lee features in the alpine and upper treeline. These avalanches were breaking 20-30 cm deep. On Saturday, several large (size 2) explosive-triggered avalanches were reported from operators, breaking 20 cm deep in the storm snow and including two cornices.

On Wednesday, a large (size 2) human-triggered avalanche releasing on the February 22 surface hoar was reported.  

Reports from earlier in the week include a natural storm and wind slab avalanche cycle up to size 2, primarily on northeast aspects in the alpine and upper treeline, releasing on the February 22 interface. This MIN post includes a photo of a skier triggered wind slab.

Snowpack Summary

20-30 cm of snow has accumulated since Friday. Strong winds from the south switched to the northwest and decreased. The winds redistributed the storm snow into a complex pattern of reactive wind slabs on lee features at upper elevations and rapidly loaded cornices. Periods of sun on Sunday are expected to initiate pinwheeling and rollerballs from strong solar radiation on steep, sunny slopes. Ice climbers should monitor for the effect of solar radiation on overhead snow and for the potential for small loose wet avalanches to have consequences.

Buried 40-70 cm deep, a weak layer of sugary faceted snow and feathery surface hoar from February 22 may persist and warrants careful evaluation. On south-facing slopes, this weak layer may be combined with a melt freeze crust. Check out the latest forecaster blog that offers a deeper dive into these conditions.

Faceted snow and crusts exist near the base of the snowpack in much of the region. This layer is has been dormant recently.

Terrain and Travel

  • Recent wind has varied in direction so watch for wind slabs on all aspects.
  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • Be aware of the potential for large avalanches due to the presence of buried surface hoar.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.